Add missing host_prep_tasks for memcached
Add missing host_prep_tasks for memcached

I missed this in change
Iecce2820657c5ad6bd2b02849c0433e5dce8009b. Memcached seems to be the
only service which mounts the /var/log/container/<service-name>
directory directly under /var/log in the container.

Change-Id: I26928bc7ad5f2f1bed60d200e8f2f0c310a14ee5
Partial-Bug: #1730957

dprince/tripleo-heat-templates,openstack/tripleo-heat-templates,openstack/tripleo-heat-templates,dprince/tripleo-heat-templates